A laser welding method and an associated device have been disclosed, for example, in German patent publication, serial number, DE 196 35 843 A1.
According to the above publication, for deep laser welding the material to be welded is locally heated to an evaporating temperature such that a steam capillary is formed in the workpiece due to the steam pressure. For generating the welding seam, the capillary is moved through the workpiece. The welding depth is determined by the extension in depth of the steam capillary in the material. Instabilities in the steam capillary can produce defects such as pores or bubbles. These defects that can occur during a laser welding process can be detected by a series of conventional methods and sensors. If a certain defect tolerance threshold has been exceeded, the defective component is characterized as a reject and is removed from the production process and is manually examined and subsequently either scrapped or repaired. For this reason, for example in motor vehicle body manufacture, the welding seams are overdetermined to prevent a component from being automatically rejected due to defective welding points.
The laser welding process is usually monitored by one of two different types of sensors: (1) light-sensitive detectors that can detect optical signals from the processing zone at different wavelengths, e.g., back reflected laser radiation and thermal radiation; and (2) camera systems that can be used for image recognition of the joining zone, in front of, within, and behind the effective area of the laser. Camera observation and light section methods permit analysis of the seam geometries.
For monitoring the laser welding process using light-sensitive sensors, a part of the processing light that is reflected by the workpiece back to the processing head or thermal radiation of the melting bath is decoupled, e.g., via beam splitters, and guided to the light-sensitive sensor. Alternatively, the sensor may be disposed outside of the laser beam next to the processing optics. Defined narrow frequency bands are detected and analysed. The intensity of the reflected radiation is a signal for the welding depth. The intensity of the reflected radiation greatly fluctuates in case of disturbances of formation of the welding seam and is an indicator of welding seam defects. The infrared emissions generated at the welding seam after welding may also give information about the welding seam quality.
Geometrical values such as the length, width, and position of the center of gravity of the melting bath and of the laser interaction zone, joint position, and steam capillary may also be determined from a camera image of the melting bath. The length or surface area of the melting bath gives information about the welding depth, since in case the laser beam penetrates deeper into the material, the melting bath length or surface area increases. Due to inclined mounting of the camera on the processing head, the process may be observed at a certain angle. A change in the focal position due to variation of the distance between the processing head and the workpiece can be clearly recognized in the camera image as displacement of the laser interaction zone.
In welding for serial production, the process signals are usually monitored and evaluated through a comparison with reference signals. Towards this end, the average reference signals are determined from some tested master weldings and are stored in the process control. Conditions are additionally determined according to which the likelihood of a welding defect is calculated from a deviation between the measured signals and the reference signals. If a certain signal limit is exceeded, a welding defect is detected and registered.
For example, the following types of welding defects can be detected in this manner:                fluctuations of the welding depth;        pores in the welding seam;        eruptions to holes (important for galvanized sheet metals in the automotive industry);        connecting defects of an I-seam at the overlap joints, where the gap is too large;        positioning errors of a joint; and        variations in the welding seam width.        
The above-mentioned document, DE 196 35 843 A1, discloses a laser welding device, in which the quality of the welding seam is permanently monitored using a high-resolution control camera and an eddy current control sensor. If a welding defect is detected, the defective welding seam is re-welded using a second laser welding apparatus. The welding parameters for this additional welding are programmed via an impulse transmitter such that the image provided by the control camera shows a geometrically impeccable welding seam. However, re-welding cannot eliminate any welding defect that might occur during laser welding, with the consequence that the defective component must then be removed from the production process as a reject.